The resolution of a microscope depends substantially on the wavelength of the illumination light used. Conventional microscopes are operated with light in the visible wavelength region (abbreviated “VIS”). In order to resolve extremely small structures, for example on wafers or circuits in the semiconductor industry, imaging at shorter wavelengths in the deep-ultraviolet region of the light spectrum (abbreviated “DUV”) is required. The microscope image is made visible using a TV camera that is sensitive to the DUV light.
The materials in conventional VIS optics are not transparent to DUV. DUV operation therefore requires optics constructed from special materials, for example prisms, beam splitters, and tube lenses, as well as objectives that are corrected for DUV wavelengths. In order to meet future requirements of the semiconductor industry, a DUV microscope preferably has capabilities for switching between VIS and DUV optics and the respective associated illuminating systems, in which a switchover between VIS and DUV objectives is also made.
In addition, the autofocus function of a microscope is an essential requirement in the semiconductor industry, since automatic (and therefore more rapid) focusing can considerably increase the number of features examined per hour as compared to manual focusing.
Known IR laser autofocus systems are greatly superior in this context, because of their higher focusing speed, to other autofocus systems (e.g. TV autofocus systems) which operate at the particular wavelength being imaged. The autofocus wavelength is shifted into the IR wavelength region so that the IR autofocus light can easily be coupled in, for example via a dichroic splitter in the imaging beam path. This prevents any loss of portions of the illuminating or imaging beam in the VIS or DUV/UV wavelength regions.
The ability for a DUV microscope also to be operated with a fast IR autofocus system is therefore a pressing need in the semiconductor industry. IR autofocus-capable objectives already exist for the VIS region, but not yet for the DUV region. In particular, a well-corrected DUV objective with high magnification and a large aperture is necessary in order to image extremely small features.
DE 39 15 868 C2 describes a 100×/0.87 DUV objective with a focal length of approximately 1.5 mm and a numerical aperture of 0.87. It comprises at least eleven lenses, which constitute a front lens group and a rear lens group. The rear lens group is configured as a shifting element. By displacing the shifting element, it is possible to change the air gap from the front lens group and thus adjust the usable wavelength region in the DUV and in a portion of the visible spectrum. The objective's correction for spherical aberration, however—for example at the common DUV illumination wavelength of 248 nm—is not particularly good. The greatest disadvantage, however, is that the objective does not have a parfocal focus in the near infrared wavelength region (abbreviated “IR”), i.e. at IR wavelengths>760 nm. The objective is therefore not suitable for use on a DUV microscope having an IR laser autofocus apparatus that works with IR wavelengths.